[Swift] Add ModalContainer to Samples

[JustinDSN]

Swift samples don't provide guidance on how to display .fullScreen modals in Workflows.

This ticket is to address one of comments of square/workflow-swift#143, and pave the way to creating a proving grounds for and modals and perhaps alerts and maybe even cards. But first things first!

@bencochran Tells me Swift's modal container isn't even in github yet – but this will be something we need to ensure is addressed when that day comes.

We can extract our internal version of WorkflowModalContainer and remove all unnecessary private bits.

Checklist:

• Extract WorkflowModalContainer into swift/Samples/ModalContainer
• Make a podspec for this new sample (as it can be used in other samples just like BackStackContainer in swift/Samples/BackStackContainer)
• Create or extend an existing sample to consume this new pod.

[JustinDSN]

Resolved by #1090